Ischemia-reperfusion injury (IRI) is a common cause of acute kidney injury (AKI) and contributes to patient morbidity and mortality in a variety of clinical settings ranging from cardiovascular surgery to renal transplantation. Recovery of renal function following AKI is often incomplete and many patients develop chronic kidney disease or end-stage renal disease. At present, little is known about the factors that facilitate the repair and regeneration of renal tubules following IRI. The lack of such knowledge has limited the development effective pharmacologic therapies for AKI. Our long-term goal is to elucidate the mechanisms involved in epithelial cell survival and proliferation during renal tubular regeneration following IRI. The objective of our current proposal is to establish a new paradigm in which heterotrimeric G protein function in renal epithelial cells is modulated by Activator of G Protein Signaling 3 (AGS3) to promote tubular cell survival and proliferation during recovery from IRI. Heterotrimeric G proteins are master molecular switches that can facilitate the activity of numerous intracellular pathways, including those involved with cell survival and proliferation. The activation/inactivation state of heterotrimeric G proteins was long believed to occur exclusively at the level of G protein-coupled receptor (GPCR) stimulation. However, the novelty of AGS3 is its ability to control heterotrimeric G protein function through an unconventional GPCR-independent mechanism. Elucidation of the mechanisms by which AGS3 and its downstream effectors modulate renal repair could be the first step in the development of a novel therapy for AKI. Little is known about the role of AGS3 in the kidney under both normal and pathologic conditions. Recently, we have made the following preliminary observations regarding AGS3 in the kidney: 1) AGS3 protein expression was markedly increased in the kidney during the recovery phase following renal IRI;2) AGS3 expression was localized predominantly to the regenerating proximal tubules in the outer stripe of the outer medulla;3) genetic knockdown of endogenous AGS3 in renal tubular epithelial cells significantly reduced cell number in vitro;and 4) renal tubular recovery following IRI was impaired in mice exhibiting partial or complete loss of AGS3 compared to wild-type control mice. Based on these observations, our central hypothesis is that AGS3 contributes to renal epithelial cell survival and tubular regeneration following IRI through a G protein-coupled receptor-independent mechanism. To address this hypothesis, the following aims were designed to: 1) define the relationship between AGS3 expression and renal tubular epithelial cell survival and proliferation during recovery from renal IRI in mice;2) determine whether genetic removal of AGS3 will impair tubular regeneration and recovery of renal function following IRI in mice;and 3) determine whether AGS3 influences renal epithelial cell number by altering the balance of proliferative and apoptotic pathways in a Gbg- dependent manner. In summary, the expected outcome from this proposal will be the establishment of novel paradigm in which AGS3 functions as a novel mediator of renal tubular regeneration in AKI. These studies are expected to have a positive impact to the field of AKI in two ways: 1) fundamentally advancing the field of renal tubular repair and regeneration by uncovering a novel regulator in this process;and 2) identification of a new therapeutic target for drug development directed towards AGS3 or its associated G proteins and downstream signaling molecules. PUBLIC HEALTH RELEVANCE: Ischemia-reperfusion injury (IRI) is a common cause of acute kidney injury (AKI) and contributes to patient morbidity in a variety of clinical settings ranging from cardiovascular surgery to renal transplantation. Currently, there are no effective pharmacologic therapies for AKI. In order to develop new therapies for AKI, it is necessary to elucidate endogenous pathways that promote repair and regeneration of injured tubules. This proposal is based on our recent finding that Activator of G protein signaling 3 (AGS3), a novel receptor- independent regulator of heterotrimeric G proteins, is upregulated in renal tubular epithelial cells during the repair phase following IRI. We will study AGS3 knockout mice to provide genetic evidence that AGS3 plays a unique role in renal tubular regeneration following ischemic injury. Moreover, we will dissect the signaling mechanism by which AGS3 functions in renal epithelial cells during the regenerative process. In all, this proposal will study a novel receptor-independent regulator of heterotrimeric G proteins and has the potential to identify a new therapeutic target in AKI.